This application is based on and incorporates herein by reference Japanese Patent Application No. 2000-399769 filed Dec. 28, 2000.
The present invention relates to a hybrid-magnet DC motor comprising stator windings and permanent magnets as part of a stator.
A small motor with high torque and low power consumption utilizing permanent magnets is widely used in various fields such as automobile, OA equipment, vending machine, and medical and welfare equipment.
In JP-A-2000-150228, it is proposed to construct a stepping motor utilizing a hybrid-magnet which comprises an electromagnet and a permanent magnet. This motor has a core with its ends bent at right angle for directing a flux of the exciting coil of the electromagnet to a stator. However, bending a path of flux reduces efficiency of the motor as efficiency of the flux is reduced.
The present invention therefore has an objective to provide a hybrid-magnet DC motor that produces high torque and low cogging torque in starting the motor. This is accomplished by effectively utilizing fluxes of electromagnets and permanent magnets in the hybrid-magnet.
According to the present invention, a hybrid-magnet DC motor comprises a cylindrical yoke, a stator including a plurality of electromagnets and permanent magnets, and a rotor including an armature. The electromagnets are placed in the yoke in a manner that opposite polarities alternately appear in a circumferential direction of the yoke. The permanent magnets are sandwiched between stator cores as their mating surfaces have the same polarities. An armature core having a plurality of pole teeth is integrated to a rotor shaft so that it rotates with a rotor shaft.
When the electromagnets and the armature are not energized, a flux of the permanent magnet forms a closed magnetic circuit within the stator. When the electromagnets and the armature are energized, fluxes of the permanent magnet and the electromagnets flow through the armature core in the same direction via the pole teeth forming closed magnetic circuits.
When the electromagnets and the armature are not energized, the cogging torque at the start of the motor is approximately zero since the fluxes of the permanent magnets form closed magnetic circuits within the stator, namely, do not flow through the armature core. On the other hand, when the electromagnets and armature are energized, strong fluxes, the sum of electromagnet flux and permanent magnet flux, are generated and high torque is produced in starting the motor.